THE 2ND INTERNATIONAL STANDARD FOR HUMAN PITUITARY LH - ITS COLLABORATIVE STUDY BY BIOASSAYS AND IMMUNOASSAYS

The second International Standard for Human Pituitary LH (in ampoules coded 80/552; 2nd IS) and LH 81/535 (prepared in the same way as the 2 nd IS from the same LH preparation) were compared with the Internation al Reference Preparation of Human Pituitary LH for Immunoassay (IRP 68 /40) by 19 laboratories in 11 countries, using in-vivo and in-vitro bi oassays, a receptor assay and immunoassays. Geometric mean estimates o f the LH content of, the 2nd IS (with 95% fiducial limits) in terms of IRP68/40 were: 34.6 (29.1-41.0) IU/ampoule by in-vivo bioassays; 35.8 (27.0-47.4) IU/ampoule by in-vitro bioassays; 58.6 IU/ampoule by one receptor assay; and 36.8 (35.5-38.1) IU/ampoule by immunoassays. The c lose agreement between the relative activities of the 2nd IS and IRP 6 8/40 in the wide range of assay systems studied appears to reflect the fact that both standards contain highly purified LH with similar isof orm compositions as judged by isoelectric focusing. Estimates of the L H content of LH 81/535 in terms of IRP 68/40 and in terms of the 2nd I S tended to be lower than those for the 2nd IS across all methods, but the differences were not statistically significant. The 2nd IS was fo und to be as suitable as IRP 68/40 as a standard for the in-vitro bioa ssay and immunoassay of LH in the two serum samples studied. However, the mean estimates of serum LH in terms of either of these standards w ere more than 150% larger by in-vitro bioassays than by immunoassays a nd more than 50% larger by one-site than by two-site immunoassays. Thi s may be a reflection of the differences in the isoform composition of the highly purified LH of the 2nd IS and IRP 68/40 and that of the LH in the sera. The significant intra- and interlaboratory variability o bserved in this study for LH estimates, especially by in-vitro bioassa ys but also by immunoassays, is very pertinent to the interpretation o f published comparisons of LH bioactivity and immunoreactivity.Estimat es of the LH content of ampoules of the 2nd IS and LH 81/535 kept at e levated temperatures showed that both the 2nd IS and LH 81/535 appeare d to be adequately stable under normal storage conditions; the in-vivo and in-vitro bioactivities and receptor binding activities of LH were more sensitive than its immunoreactivities to thermal degradation of the LH structure. On the basis of these results, the World Health Orga nization Expert Committee on Biological Standardization established th e preparation in ampoules coded 80/552 as the second International Sta ndard for Human Pituitary Luteinizing Hormone, and assigned an activit y of 35 International Units of human pituitary LH to the contents of e ach ampoule.